The present invention relates generally to the field of copy protection of a digital data file and more particularly, but without limitation, to the placement of copy protection on an optical disc by substituting second data set samples for user data samples and writing induced errors therein.
Optical discs have become increasingly popular as an efficient and cost-effective storage medium for digitally stored data. A typical optical disc comprises a circular disc having a recording layer of light reflective material embedded in a refractive substrate. The recording layer is disposed along a plane substantially normal to an axis about which the disc is rotated and stores data in the form of localized pits and lands (also sometimes referred to as xe2x80x9cmarksxe2x80x9d and xe2x80x9cspacesxe2x80x9d) along a continuously extending spiral track. The length of each pit and land corresponds to one of a selected number of data symbols (for example, from 3T to 11T, with T of determined length).
The data symbols are recovered from the disc through the use of a light source (such as a laser) which applies light of selected wavelength to the rotating disc and a transducer which generates a readback signal indicative of the data in relation to the relative differences in reflectivity of the pits and lands. It is common to separate the relative elevations of the pits and the lands by a distance equal to a quarter-wavelength of the applied light so as to facilitate a substantial change in the amount of light reflected by the pits as compared to the amount of light reflected by the lands.
One popular optical disc format is commonly referred to as compact disc, or CD, which has found widespread use in recent years in computer applications (such as CD-ROM) and in the music recording industry (audio CDs). A CD has an outer diameter of 120 millimeters (4.724 inches) and a data storage capacity of about 650 megabytes (MB). Another popular optical disc format is commonly referred to as digital versatile disc, or DVD. DVDs of the current generation have the same form factor as CDs, but store about 4.7 gigabytes (GB) per recording layer.
Due to the worldwide consumer demand for the types of information available on optical discs (e.g. software and music), combined with the relative ease with which unauthorized copies of optical discs can be generated, suppliers of optical discs have attempted to implement various copy protection schemes to restrict unauthorized replication of the disc. One type of copy protection involves configuring an optical disc in such a manner so that an authorized copy functions properly in a readback system, but an unauthorized copy is prevented from doing so. Another type of copy protection provides a forensic tracking scheme by storing certain xe2x80x9cmarkingxe2x80x9d or xe2x80x9ctrackingxe2x80x9d information on the disc, with this information relating to the source of the disc, the mastering data and so on. The forensic information, generally, does not prevent an unauthorized copy from functioning in the readback system, but based on the presence or absence of the information, a determination can be made whether a particular optical disc is in fact an authorized copy.
One particularly common copy protection scheme for CDs involves manipulation of the error correction system. As will be recognized by those skilled in the art, CDs generally utilize three levels of error correction, referred to as C1, C2, and Level 3. The C1 codes are parity words determined from the user data, the C2 codes are error correction codes (ECC, such as Reed-Solomon codes) calculated from the user data and the C1 codes, and Level 3 are top-level parity words. These levels of error detection and correction codes are accordingly calculated and stored as the user data are written to the discs and each provide successively higher levels of error detection and correction capabilities for the associated user data. That is, if errors cannot be corrected from the C1 codes, the C2 codes are employed, and if the C2 codes are unsuccessful, then the Level 3 parity words correct the errors.
Purposefully writing erroneous (xe2x80x9cbadxe2x80x9d) C2 codes at a few selected locations on the disc will result in a particular signature that would not normally occur during normal C2 calculation steps. Thus, if the optical disc is a CD-ROM, an active application resident in a computer can first check the CD-ROM to ensure the bad C2 codes are present at the selected locations to verify that the CD-ROM is an authorized copy. If the C2 codes are not bad at the selected locations, the application can report an error and prevent the user from using the CD-ROM.
While operable, there are disadvantages with this type of copy protection approach. Manipulating the error correction system results in some degradation of the error correcting capabilities of the disc, at least potentially making the disc unusable if enough errors arise over time that cannot be adequately corrected using the C1 and Level 3 codes.
Such manipulation also results in optical discs that technically do not meet the various CD industry format standards, and accordingly, the bad C2 codes are easily detectable by standard test equipment used to verify optical disc replicas. Thus, during manufacturing by an authorized replication source, the detected errors will be reported for every replicated disc. These reported errors will have to be investigated to determine whether the errors are valid errors, or errors due to the copy protection scheme (and should thus be ignored). To increase the efficiency of the testing of optical discs having these C2 errors, often times test equipment are instructed to ignore specific sections of the optical disc, as many of the errors are intentional, and not due to manufacturing errors. When this occurs, any actual errors are overlooked, therefore the integrity of the optical disc is jeopardized by this copy protection scheme.
More importantly, because the bad C2 codes are easily detected, an unauthorized source can insert bad C2 codes in the appropriate locations in the master disc and defeat the copy protection scheme altogether. Since audio CDs do not use a resident computer application that can initially verify the authenticity of the disc, this type of protection scheme is usually not applicable to audio CDs.
When a reading device encounters an error, it attempts to correct the error using the resident error correction codes. Should this error be of such magnitude that the error correction codes are incapable of reproducing the correct signal, the optical reading device works to conceal the error. The error in an audio CD, without concealment, may result in an audible click and disrupt the enjoyment of the audio track. The error concealment system, usually incorporated within the digital to analog converter of the optical reading device, employs interpolation and muting processes to conceal the error. The system is notified of the error by an analog correction flag produced by the demodulating system, and based on several factors, the system makes a determination of how to most effectively conceal the error. The system may interpolate the last known value and the next known value to approximate the missing value, mute the output signal for the duration of the error using the players internal muting method, or simply hold the last known value for the duration of the error. All these techniques are implemented to provide an inaudible concealment of the uncorrectable error.
Due to the disparity in how different optical discs are accessed, some having a resident computer application and others simply processing the encoded data, there is a need for a copy protection scheme which prevents the unauthorized duplication of an original disc by a typical mastering or recording device but does not otherwise interfere with the output of the user data, such scheme operable in different types of readback systems.
The present invention is directed to a copy protection scheme for the prevention of the unauthorized duplication of user data without adversely affecting the integrity of the user data itself. More specifically, the present invention is directed to the placement of a copy protection scheme on an optical disc to prevent unauthorized duplication of the optical disc by a typical mastering configuration or recording device. The copy protection scheme is discussed as applicable to an optical disc, and more specifically an audio CD, but the present invention is fully applicable to any data set capable of being digitally represented and having error correction codes incorporated therein.
The copy protection scheme is created by superimposing (intermingling) a second data set with the user data at specific locations by substituting data samples of the second data set in place of user data samples. At the location where the second data set samples are written over and replace the user data samples, an uncorrectable error (CU) is written.
When a reading device reads the user data and encounters the CU, it will be instructed to ignore the data sample written therein as being incorrect and will activate its internal error concealment steps to compensate accordingly. Therefore, the output of the user data signal will be an almost exact replication of the original signal prior to the substitution of the second data set therein with no intelligible distortion to the output signal due to the second data sample being concealed.
In the case of an optical disc reader, when the reader encounters an uncorrectable error, the error concealment system can hold the last sample until the next readable sample, mute the output until the next readable sample, or interpolate the sample based on the value of other samples. All of these CU compensation operations occur with minimal interference to the audio output.
When the user data are illegally copied, the copy protection scheme of the present invention prevents the reconstruction of the original signal. In the extraction of the user data for recording purposes, the CUs are not made a part of the output signal and the error concealment steps are not taken. Therefore, the second data set samples substituted into the user data set are made a part of the original signal without any internal notification for a reading device to ignore or conceal the data set sample values. An output signal which is distinctly different from the original user data signal is created. In the case of an audio disc, the audio output may comprise the original data (e.g. songs) with a disruptive second track of sounds superimposed thereon, such as a high frequency warble, a series of clicks, or a human voice which states that this is an unauthorized disc.
Other features and advantages which characterize the present invention will be apparent from a review of the following detailed description section in conjunction with the appended drawings.